The aim of the present investigation is to study fatty acid metabolism by fetal heart and liver and the influence of hormones and pharmacological agents on this metabolism. This is carried out at the intact organ, subcellular and tissue culture level. The fatty acid synthesizing systems of chick embryo liver could be prematurely induced by administration of insulin, hydrocortisone, glucagon, growth hormone or cyclic AMP. The similar effects of two classical antagonists, insulin and glucagon, suggested the possibility that the membrane receptors for these hormones are not differentiated in embryonic liver plasma membranes. Embryonic heart has the capacity to synthesize fatty acids but loses this capacity as it develops. The heart then depends on blood fatty acids for energy and structural requirements. It is planned to investigate: (1) de novo fatty acid synthesis and mitochondrial and microsomal chain elongation systems in cell cultures of heart and liver and determine whether the three systems are inducible by hormones; (2) the contribution of the reversal of mitochondrial Beta- oxidation to the fatty acid synthetic capacity of embryonic heart, in ovo and in tissue culture; (3) the lipid requirements of the culture media for the growth of heart and liver cells; and (4) the uptake of fatty acids by heart cells in culture.